1. Field of the Invention
The present invention relates to a solid state image pick-up apparatus, and more particularly to a color solid state image pick-up apparatus comprising a color separation optical system for dividing an image of an object to be picked-up into red, green and blue color images and a plurality of solid state image sensors for receiving these color images.
2. Description of the Related Art
In the solid state image pick-up apparatus, a charge coupled device (CCD) has been generally used as a solid state image sensor. CCD cameras have been widely used, not only in video cameras for domestic use, but also in television cameras for broadcasting use. In order to improve the resolution of the CCD camera, a lot of work has been done for increasing the number of light receiving elements in the CCD. However, this approach is limited by to a limitation in miniaturization. Further, when the number of light receiving elements is increased the aperture of each element is reduced, so that S/N, sensitivity and dynamic range are liable to be decreased. Therefore, each aperture of the light receiving element could be reduced only to a limited extent.
In order to mitigate the above mentioned drawback, a so-called offset-site pick-up method has been proposed. FIG. 1 illustrates a known color television camera using the offset-site pick-up method. An image of an object to be picked-up is formed by an objective lens 10 and the thus formed image is separated by a color separation optical system 11 into red, green and blue color images. These color images are picked-up by means of respective solid state image sensors, i.e., CCDs 12R, 12G and 12B. As shown in FIG. 2A, light receiving elements of the CCDs 12R and 12B receiving the red and blue color images, respectively, are shifted with respect to light receiving elements of the CCD 12G viewed in a main scanning direction, i.e., in a horizontal scanning direction substantially by a half of a pitch P at which the light receiving elements of these CCDs are arranged regularly in the horizontal scanning direction.
Color image signals read out of CCD 12R, 12G and 12B, respectively, are supplied to correlated double sampling circuits 13R, 13G and 13B, respectively, to derive red, green and blue image signals. In order to make the phases of these color image signals identical with each other, the green image signal is supplied to a delay circuit 14 having a delay time .tau. which corresponds to a time interval during which a distance of a half pixel pitch is scanned. The red, green and blue image signals having the identical phases are supplied to low pass filters 15R, 15G and 15B, respectively, to remove high frequency sampling noise, and then are supplied to image processing circuits 16R, 16G and 16B, respectively. The red, green and blue image signals are further supplied to variable delay circuits 17R, 17G and 17B, respectively, to compensate remaining small phase differences.
The thus obtained color image signals are supplied to a matrix circuit 18 and are combined therein at a given ratio to derive a luminance signal Y. In NTSC standard, the red, green and blue image signals are mixed at a ratio of 0.30:0.59:0.11. Then, the thus obtained luminance signal Y is amplified by an amplifier 19. In this manner, in the color television camera using the offset-site pick-up method, the color image signals are spatially sampled such that the red and blue pixels R and B are arranged between adjacent green pixels G as depicted in FIG. 2B, so that the resolution of the luminance signal Y obtained by mixing these image signals is apparently increased twice and a folding alias component which might deteriorate the quality of a reproduced image is reduced.
In the known color television camera using the offset-site pick-up, as far as a single color channel is concerned, the number of pixels of a CCD is not increased at all, and thus the resolution is not increased and the folding alias component or pseudo signal is not decreased. This is due to the fact that a spatial low pass filter, i.e., optical low pass filter arranged between the objective lens 10 and CCDs is designed such that the maximum effect could be attained for the luminance signal Y obtained by the offset-site pick-up. This will be explained further.
In CCD having 400,000 pixels, the sampling frequency is about 14 MHz. When this CCD is used in the offset-site pick-up camera, the sampling frequency is increased twice and becomes about 28 MHz. Therefore, if a low pass filter having a cut-off frequency which is a half of the sampling frequency is provided before or after the sampling circuit, there is not produced the pseudo signal owing to the Nyquist theory. Therefore, in the known color television camera comprising three CCDs, the spatial low pass filter is designed to cut off a spatial frequency component higher than about 14 MHz, so that the pseudo signal is suppressed in the luminance signal. This will be explained further in detail with reference to FIG. 3.
As explained above, in the offset-site pick-up camera, the sampling frequency is apparently increased twice as compared with a single CCD camera. In a CCD having 400,000 pixels, the response is increased up to twice the sampling frequency, i.e., about 28 MHz. FIG. 3A illustrates the response of the luminance signal Y and alias component when the spatial low pass filter is not provided. In this case, the folding alias component appears largely in the lower frequency region. FIG. 3B shows the response of the spatial low pass filter having the cut-off frequency of 14 MHz, and FIG. 3C depicts the response of the luminance signal Y and pseudo signal when the signal having the response shown in FIG. 3A is passed through said spatial low pass filter. When the spatial low pass filter is used, the pseudo signal is reduced to a large extent. However, for respective channels, the sampling frequency is the same as the single CCD camera, i.e., 14 MHz, and thus the frequency response of the color signal and alias component is represented in FIG. 4A. It is assumed that the solid state image sensor has the aperture ratio of 50%. Therefore, when the spatial low pass filter having the cut-off frequency of 14 MHz is inserted, the frequency response becomes as shown in FIG. 4B. That is to say, the alias component is hardly reduced. In this case, in order to reduce the alias component in respective color channels, it is necessary to provide a spatial low pass filter having a cut-off frequency of 7 MHz. This 7 MHz corresponds to 560 TL (television line), so that the resolution is reduced lower than 560 TL.
In the NTSC standard, the luminance signal Y is derived by mixing the red, green and blue color signals at the ratio of 0.30:0.59:0.11, and therefore when the offset-site pick-up is adopted between the green channel and the red and blue channels, the mixing ratio of the color signals becomes 0.59:(0.30+0.11)=0.59:0.41. In this manner, the mixing ratio deviates from 1:1. Then, the offset-site pick-up becomes out of order by an amount which corresponds to a deviation of the mixing ratio from the ideal mixing ratio of 1:1. That is to say, the effectiveness of the offset-site pick-up is reduced by about 20%.
Further, as shown in FIG. 1, there are provided the low pass filters 15R, 15G, 15B and image signal processing circuits 16R, 16G, 16B between the CCDs 12R, 12G, 12B and the luminance matrix 18, so that the color image signals are liable to be affected by the deterioration in the frequency characteristic and the deviation in time delay between the color image signals which affect the effectiveness of the offset-site pick-up. For instance, in case of using the CCD of 400,000 pixels, the resolution is limited to about 700 TL. Particularly, when the distance between the CCD and the signal processing circuit is long, deterioration in the frequency characteristic and deviation in time delay are introduced, and thus the effectiveness of the offset-site pick-up might be further lost. In this manner, in the known television camera using the offset-site pick-up, it is necessary to strictly manage the time of the color image signals between the CCDs and the luminance matrix, and if there is a time deviation of 10 ns between the color image signals derived from the CCD of 400,000 pixels, the effectiveness of the offset-site pick-up is no longer attained. Therefore, the deviation in time has to be suppressed below this value, but this could be realized only with difficulty.
Furthermore, the objective lens has chromatic aberration and distortion, so that there is a so-called registration error produced between the red, green and blue color image signals, which also affects the effectiveness of the offset-site pick-up. A zoom lens is generally used as the objective lens for the television camera. Then, large aberrations are produced. That is to say, there are produced an on-axis chrominance aberration in which an imaging position is varied in accordance with a zooming ratio and a multiplication chromatic aberration in which the imaging magnification is changed for respective color images in a periphery of the image. Therefore, a very large registration error is produced and the function of the offset-site pick-up is further reduced. Such a disadvantage also occurs when the registration error is generated by a de-focused condition.
In the NTSC standard, it is recommended that the objective lens have a registration error smaller than 10 .mu.m. However, in a newly developed high definition television, it is desired to make the registration error smaller than 5 .mu.m. However, when a 1/4 inch CCD having 1,400,000 pixels is used, since the pixel pitch P is 7.6 .mu.m (H).times.5.2 .mu.m (V), the amount of the pixel shift has to be set to 3.8 .mu.m and thus when the objective lens has a chrominance aberration of about 5 to 10 .mu.m, it is no longer possible to obtain the effective offset-site pick-up over the whole image, but the offset-site pick-up could be effective only in a limited condition. For instance, the offset-site pick-up is effective only in the central portion of the image or at a middle of wide and telescopic ends or for F4.0.
In order to mitigate the above mentioned drawback, there has been proposed a CCD camera using a so-called dual-green system. In this known camera, two CCDs are arranged to receive the green image and are shifted in the horizontal direction by a half of the pixel pitch and one CCD is used to receive the red and blue images which has a stripe or mosaic red and blue color filter applied thereon. In this case, as far as the green color signal is concerned, the ideal offset-site pick-up can be obtained and the above mentioned drawback can be overcome, but the resolution of the red and blue color signals is reduced by two.
In Japanese Patent Application Laid-open Publication Kokai Sho 60-154781, there is described a color television camera, in which the image of the object to be picked-up is separated into two green images, one red image and one blue image and these four images are received by four CCDs which are arranged in the offset-site pick-up mode. However, in such a known camera, the color separation optical system is liable to be complicated in construction and large in size, so that the distance between an objective lens fixing surface to the light receiving surfaces of CCDs (generally termed flange back) becomes long and might be beyond a predetermined value. In such a case, existing objective lenses could not be utilized. This is due to the fact that in the color separation optical system, it is necessary to avoid undesired right and left reverse due to the odd numbered reflections. Moreover, in the known camera, the offset-site pick-up is performed between different colors, and it is affected by the chrominance aberration of the objective lens, so that the offset-site pick-up is effective only in a very narrow area.
When the CCD having 1,300,000 pixels is used in the dual green camera, 1250 pixels are arranged in a horizontal line, so that the number of pixels in one line of each of the red and blue color signals is 625 pixels. The resolution of this CCD is 700 TL, so that for the red and blue images, the resolution amounts to only 350 TL. In the current high-vision broadcasting, sampling frequencies for digital equipments in a broadcasting studio are determined to be 74.25 MHz for the luminance signal and 37.125 MHz for the color difference signals. Therefore, for the luminance signal, an apparent sampling frequency becomes 48.6.times.2=97.2 MHz which is higher than the sampling frequency for the digital equipments connected to the image pick-up apparatus, but for the color difference signals the apparent sampling frequency is determined by the bandwidth of the red and blue color signals and becomes 24.3 MHz which is lower than the sampling frequency of the digital equipments.
Moreover, for the red and blue channels, there is provided only one CCD having the stripe or mosaic color filter applied thereon, so that if the horizontal transporting efficiency within CCD is low, the red and blue color signals are mixed with each other and the color separation is deteriorated.